Symptoms of long COVID experienced by children and young people change over time and while some of the children's original symptoms declined, new symptoms were reported, the world's largest study on long COVID in children published in the Lancet journal has revealed.

This was also the case when researchers looked at scales measuring the poor quality of life, experiencing emotional and behavioural difficulties, poor well-being, and fatigue.

"Our research goes one step further than existing studies and indicates that researchers need to track individual trajectories using repeated measurement on the same children and young people over time," said Snehal Pinto Pereira from University College London in the study published in The Lancet Regional Health.

Researchers asked children aged 11 to 17 about their health six months and 12 months after taking a PCR test between September 2020 and March 2021. They also asked them to recall their symptoms at the time of taking the test.

Researchers asked the children and young people what they had experienced from a list of 21 symptoms, including shortness of breath and tiredness, as well as using validated scales to assess the quality of life, mental health, well-being, and fatigue.

The team found that, at the time of testing, health issues were more common in children and young people who had tested positive for the virus compared to those who tested negative, as well as six months and 12 months post PCR test. They noted that the symptoms experienced changed over the course of a year.

"Simply reporting repeated cross-sectional prevalences -- or snapshots -- of symptoms over time may obscure important information about long COVID in young people that has clinical relevance," said Pereira. The landmark study will continue to analyse survey results from participants for up to two years after the initial PCR test.